Broadband SNR Margin Tweak

Signal to Noise Ratio (SNR) margin tweaking allows broadband users to fully take control of their ADSL or ADSL2+ broadband connection to squeeze out the very highest speeds possible. With SNR margin tweaking, your downlink broadband speeds are no longer dictated by equipment (shown above) in the BT exchange. SNR margin adjustment could more than double your downlink connection speed on its own! When combined with some of our other recommendations, such as an ADSL faceplate and a better modem router, you could get even greater broadband speed improvements.

What is the ‘target SNR margin’?

The target Signal-to-Noise Ratio (SNR) margin is particularly important in defining your broadband download speed. It effectively defines the buffer required in signal-to-noise ratio to protect your broadband connection from the fluctuations that can occur over time in signal and noise levels. Poorer lines tend to require higher target SNR margins to avoid regular disconnections.

The exchange equipment attempts to set the downlink connection speed such that there is a sufficiently high SNR margin to guarantee a reliable connection (without regular drops and/or an unacceptable number of errors). This specific margin that the exchange tries to achieve is called the target SNR margin. The higher the target SNR margin, the lower your connection speed.

As we will discuss, since a modem measures the downlink SNR and reports this measurement back to the BT exchange equipment (which then sets the downlink connection speed), it is possible (with some modems) to adjust this measurement so that the BT exchange equipment believes that the SNR is higher than it really is so that it sets a higher connection speed than it would have done.

BT can increase your downlink target SNR margin to 18 dB

When your broadband service is first switched on, the target SNR margin is set by the BT exchange equipment at 6 dB. On a good line, BT generally considers that a 6 dB SNR margin is appropriate to maintain a connection without a significant number of drops or errors. The corresponding target margin for TalkTalk equipment is 9 dB.

As part of the Digital Line Management process, the exchange equipment continuously monitors your broadband connection, for example recording the number of times that it drops and the number of errors it suffers. If the line experiences any issues (such as bursts of errors at a particular time), the exchange equipment can increase the target SNR margin – usually in steps of 3 dB. This action – aimed at stabilising the line – reduces the connection speed.

BT’s DLM system, or BT’s manual intervention, can increase the target SNR margin to a maximum value of 18 dB.

Many things can cause the exchange equipment to increase the target SNR margin, including:

not using a filtered faceplate (see our ADSL faceplate page for more information)

a faulty broadband microfilter, or forgetting to install a microfilter on all extension phone sockets.

Once the BT exchange equipment has increased the target SNR margin, it can take days for the target SNR margin to be reduced and, in some cases, it is never reduced. In the worst case scenario, you may be saddled with very slow downlink speeds caused by an 18 dB target SNR margin when your line could work perfectly with a 3 dB (or less) SNR margin. We will show you how to adjust your target SNR margin to any value that you desire. You will no longer be a slave to Digital Line Management in your exchange!

Relationship between downlink speed and target SNR margin

To demonstrate the impact of different target SNR margins on downlink connection speeds, here are some results from an ADSL connection based in rural Cambridgeshire. With the modem used (the Billion 7800N), it has been possible to demonstrate how the downlink connection speed is affected by the target SNR margin.

In this example, the downlink speed is only 3008 kbps when the target SNR margin is set at 18 dB, which is substantially lower than the 5568 kbps achieved by the BT default target SNR margin of 6 dB. With this particular connection, it was possible to reduce the target SNR margin to only 1 dB, while maintaining the reliability of the connection, to achieve an amazing 6624 kbps downlink connection speed. This connection speed is 120% faster than the connection speed with an 18 dB target SNR margin.

Downlink target SNR margin (dB)

Downlink connection speed (kbps)

18 (worst case)

3008

15

3616

12

4096

9 (TalkTalk's initial default)

4768

6 (BT's initial default)

5568

3

6240

1

6624

Table: Effect of target SNR margin on downlink connection speed

Please bear in mind that lowering the downlink target SNR margin can increase the number of line drops or line errors, and not all lines with be stable at 3 dB SNR margin or lower. However, you can try to see what target SNR margin works best for your line, to get the optimum balance between reliability and connection speed. Particularly where you have made significant improvements to your line (for example, by purchasing a new modem or installing a faceplate broadband filter) your line may be able to tolerate substantial reductions in target SNR margin without adversely affecting reliability.

How can I tweak the target SNR margin?

The ability to tweak the target downlink SNR margin depends critically on the make and model of your modem. Depending on your particular model of broadband modem, there are a number of methods for tweaking the target SNR margin, which include:

connecting to the modem using telnet, and issuing the ‘adslctl configure –mod snr X’ or command.

You can easily tweak the target SNR margin with the Billion 7800DXL, 8800NL or 7800N

Of all the methods, by far the easiest is to purchase one of the new Billion 7800DXL or Billion 8800NL modem routers (or the older 7800N model). These incorporate a very simple target SNR margin adjustment page (and full instructions are provided on our dedicated Billion 7800DXL,Billion 8800NL and Billion 7800N pages).

Target SNR margin adjustment with these modems is very simple, and the modems store the SNR settings even when they have been switched off.

If you’re considering replacing your existing modem, the Billion 7800DXL and Billion 8800NL models are perfect for broadband users wanting to get the very highest speeds from their broadband connection and we strongly recommend them.

DGTeam has develop custom firmware with SNR margin adjustment

DGTeam provides unofficial custom firmware developed for selected Netgear modems. This firmware allows the downlink target SNR margin to be adjusted (which cannot be done using the standard Netgear firmware). The firmware is free of charge to download, and versions are available for the following models:

Netgear DG834GT

Netgear DG834PN

Netgear DG834N-V1

Netgear DGN2000

Netgear DG834(G) V1 V2

DG834(G) V3

DG834(G) V4

Netgear DG834GTB (Annex B)

Netgear DG834PNB (Annex B)

We have regularly used the DGTeam firmware over a number years, and have found it very reliable and easy to use. We still, however, find that broadband connections made with our favourite Billion modems are generally more robust than with Netgear modems – with or without the DGTeam firmware.

Please note that the target SNR margin adjustment using the DGTeam firmware is limited to about -5.5 dB in the downward direction. This means that if BT has increased your target SNR margin to 15 dB, for example, you will only be able to decrease this to about 9.5 dB.

RouterStats allows you to monitor your connection stats and adjust the target SNR margin with a supported modem

RouterStats is powerful free software (which runs with Windows or Linux) that connects to your modem to provide a comprehensive range of statistics and graphs. For example, you can monitor (over time) the actual SNR margin and the number of errors experienced on your line, which can help to diagnose problems. In addition, RouterStats can be used to adjust the target SNR margin in some modems. RouterStats can be configured to work with a broad range of modems, including:

Netgear DG834

Netgeat DG834G

Netgear DG834GT

Netgear DG834N

Netgear DG834PN

Netgear DG2000

Netgear DGN2000

Thomson ST585 v6 and v7

Draytek Vigor 2600G

Linksys WAG54G

Linksys HG200

Linksys WAG160N

Linksys WAG200G

Linksys WAG354G

Netgear DM111P

Orange Livebox (Inventel)

Speedtouch 510 v4 (Firmware 4.3.2.6)

Speedtouch 585 v6 & v7

Voyager 2100

Voyager 220V

Zoom X5v

On the RouterStats website, the developers warn that the RouterStats software is not ‘plug and play’. They point out that all routers are different and users may need to do some detective work to get RouterStats working with a particular modem. We find the RouterStats software is relatively complicated to use, but if you have the time to search the Internet for support information, then it may provide a cost-effective way to change the target SNR margin.

You can tweak some Broadcom-based routers using telnet

Some Broadcom-based modems allow the target SNR margin to be tweaked using the ‘adslctl configure’ command in telnet (or ‘adsl configure’ depending on the modem).

From your PC, configure a telnet connection to your modem.

So, if your modem is set up as http://192.168.1.254 then, from the Command Prompt, type:

telnet 192.168.1.254

and then enter the login ID and password.

Then issue the following command:

adslctl configure –snr N

or

adsl configure –snr N (depending on the modem)

where N is the number shown in the table below. The optimum value of N is determined by BT’s current target SNR margin and your desired target margin.

Current target SNR margin (dB)

Desired target SNR margin (dB)

Number you should enter

6.0

4.5

75

6.0

3.0

50

9.0

6.0

50

9.0

4.5

25

9.0

3.5

1

12.0

9.0

50

12.0

6.5

1

12.0

6.0

65550

12.0

4.5

65525

12.0

3.0

65500

15.0

12.0

50

15.0

9.0

65550

15.0

6.0

65500

15.0

4.5

65475

15.0

3.0

65450

18.0

15.0

50

18.0

12.0

65550

18.0

9.0

65500

18.0

6.0

65450

18.0

4.5

65425

18.0

3.0

65400

If the table above is rather confusing, the table below presents the information in a different way. Basically, you start off with whatever your default target SNR margin value is, and work your way down the table (to increase speed) until you find the optimum setting for your line. You can always reset back to the default value by using the number 100. Note that you can use values between those shown, but we wanted to avoid having a huge table!

Change you want in target SNR margin

Number you should enter

No change/reset to default value

100

Lower by 1.5 dB

75

Lower by 3.0 dB

50

Lower by 4.5 dB

25

Lower by 6.0 dB

65550

Lower by 7.5 dB

65525

Lower by 9.0 dB

65500

Lower by 10.5 dB

65475

Lower by 12.0 dB

65450

Lower by 13.5 dB

65425

Lower by 15.0 dB

65400

Please note that this tweak will not work with some recent modems, because the ‘adslctl configure’ or ‘adsl configure’ commands have been deliberately disabled in the firmware of the modem by the manufacturers.

Also, note that the target SNR margin settings will be reset once the modem is rebooted, severely limiting its practical usefulness.

It could make a big difference to broadband speeds

We hope that you have gathered from this page that the downlink target SNR margin is a very important factor in determining your downlink connection speed. You pay a high price in terms of broadband connection speed if the target SNR margin is increased by BT’s Digital Line Management from its default 6 dB setting. We have documented a number of ways in which you may be able to reduce your downlink target SNR margin to substantially increase your speeds.

The easiest way to adjust your target SNR margin is to upgrade to a modem equipped with this feature. For example, the affordable and future-proofed Billion 8800NL supports ADSL and ADSL2+ AND supports fibre broadband (VDSL2) as well. It can be obtained here from Amazon UK at a great price. The small commission we generate from you following this link helps keep this broadband information going. Thanks!